Dell Latitude C840,Internet connection problem

I have just reinstalled WindowsXP on Dell Latitude C8408(laptop).Everything is fine except the fact that I can't connect to the internet through Lan(router).There is always a red X at the local area connection.Personally,I think the problem is with network adapters,not with the internet connection line because I can connect to internet using other computer.There are two network adapters:1394 Net Adapter and 3Com 3C920 Intergrated Fast Ethernet Controller(3C905C-TX Compatible) at the Device Manager.I searched google and saw articles where people advised to disable 1394 net adapter,I did but still can't connect to the internet.Does anyone know where is the problem?Did I forget to set up anything or do something wrong?

By the way,I lost the Driver CD enclosed with the laptop so I have to download and install manually the drivers from dell.com.The drivers I have installed till now are chipset and network drivers.Do I need to install other drivers as well?

First, the 1394 adapter is your firewire port. There is no need to disable that as it will never interfere. Second, did you install the chipset drivers as the first drivers installed after the reinstallation of Windows. If they weren't the first drivers installed, then you may need to reinstall the drivers for the network adapter, as Windows will not properly install these drivers unless the chipset drivers are present.

Although this will not effect the network, you probably also need to install drivers for your video adapter, sound card, modem, and touchpad.

If the chipset drivers were not installed first, and you cannot get it to work by reinstalling the network drivers, sometimes the only solution is to start over - reinstalling Windows then installing the drivers in the correct order.

Yes,the network card does show up in Device Manager,no problem with it but in Network Connections it shows up with a red X.

The other devices that have problems(with a yellow question mark) are PCI Modem and Video Adapter which I think there is no connection with network card.

Now I am going to open the laptop case to change the Lan network plug-in and if it doesn't work I will change the network card.I don't have any experience of repairing the laptop,even never open it before so do you think it is risky to do that?

I have the same problems except I can connect on some routers and not on other routers which is the wierd thing. Fine at home so I don't think that it is the network connector itself I have tried reinstalling everything from scratch and same thing. Anyone else have this and fix it or do I have to keep using my NIC card.